Getting the Facts About Glycaemic Index Right
Make GI Work for You
For those new to glycaemic index, or GI as it is most often referred to, it is a measure of the effect of any given carbohydrate-containing food or drink on blood glucose levels (blood sugar levels) in the body. GI is a number that ranges from 0 to 100, with 0-55 indicating a low-GI food, 56-69 medium-GI and 70-100 a high-GI food. The higher the value, the more rapid the rise in blood glucose levels. To make a fair comparison between foods they are tested against a reference substance with a GI value of 100 (the highest value possible). Traditionally the reference substance was glucose, but nowadays more often than not a 50g portion of white bread made with refined flour is used, because the glycaemic response to this tends to be more reliable.
As a general rule, complex, starchy carbohydrates, which are comprised of longer chains of glucose molecules tend to have a lower GI and foods rich in refined, starchy carbohydrates or simple sugars (e.g. sucrose, glucose or fructose) have higher values. Physiologically, what happens in the body following consumption of a carbohydrate-containing food or drink is that the pancreas releases the hormone insulin into the blood stream, which stimulates the uptake of glucose into the body's cells where it can be used for energy. Sometimes, though, this surge in insulin production is over-compensatory and there is a subsequent slump in blood glucose levels shortly afterwards, a sensation known as a sugar crash. Symptoms may include hunger, lethargy and irritability.
So, one of the proposed advantages of low-GI foods is that following digestion you are less likely to experience this drop in glucose levels; your energy levels remain more stable throughout the day and your appetite is better regulated. Consequently you experience a higher level of productivity, greater concentration, improved mood and easier weight management. Although there is some truth to this, it cannot be taken as gospel. As Anita Bean, one of the most respected sports nutritionists in the UK and author of Food for Fitness, succinctly describes " there have been no long-term studies, but of the short-term studies to date only about half have found that low-GI foods reduce hunger, increase satiety or reduce overall food intake. No difference in satiety or food intake was found in the remaining half".
Factors Influencing Glycaemic Response
As with many topics related to diet, the issue of GI is not clear cut, as many factors have an influence on the body's glycaemic response:
- Portion size - The more you eat, the greater the glycaemic response. (Glycaemic load does take this into account however, and can be calculated by multiplying the GI value of the food by the carbohydrate per portion consumed, divided by 100.)
- Food combinations - The body responds to the mixture of foods and fluids entering the stomach, for example eating a high-GI food with a low-GI food will result in a medium glycaemic response.
- Other qualities of the meal - Protein, fat and fibre in a meal all blunt the glycaemic response.
- What you had for your previous meal - Your blood sugar level at the beginning of a meal (governed by food consumed and activity levels beforehand) influences what it will be at the end of the meal.
- How active you are - Any physical activity undertaken before or after a meal will affect post-prandial blood sugar levels.
- Individual variations - Different people have different responses, e.g. visceral adiposity is closely linked with insulin resistance, meaning that those with a larger waist circumference are likely to have more pronounced rises in blood glucose levels following carbohydrate-containing meals.
All of these factors show that it is wise not to base dietary decisions on GI alone, but rather look at the wider picture. A low-GI value should not be interpreted as a license to eat freely and it certainly shouldn't cloud your evaluation of the overall nutrient content of a food.
Applying the Principles of GI
- Apply your knowledge of GI in the context of healthy eating.
- Base each meal on a staple food that is rich in starchy carbohydrate and try to opt for low-GI choices more often than not, e.g. basmati rice, bulgar wheat, pasta cooked al dente, granary and seeded breads.
- Stick to sensible portion sizes.
- Combine foods with different GIs to achieve a balanced diet.
- Stave off hunger between meals by choosing nutrient-rich, low-fat and low-sugar snacks. If they are low-GI even better, but don't necessarily make this your priority.
- Listen to your own body. Build up an idea of how different foods affect you. Know which foods keep you feeling fuller for longer, give you sustainable energy and ultimately make you feel your best, both physically and psychologically.
Resources
Bean A., Food for Fitness, (3rd edition), A & C Black Publishers, 2007.
Murray M. & Pizzorno J., The Encyclopaedia of Healing Foods, Piatkus Books, 2005.
Various contributors, (editorial by Grapevine Publishing Services, London), Collins Gem GI, HarperCollins Publishers, 2005.
Eat a range of carbohydrates for a balanced diet
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